Magnetic drill press

ABSTRACT

A magnetic base for selectively magnetically latching to a ferromagnetic surface includes a magnet holder, a fixed magnet assembly supported within the magnet holder, a movable magnet assembly supported within the magnet holder, and a transmission. The transmission includes an input, an output coupled to the movable magnet assembly for moving the rotatable movable assembly relative to the fixed magnet assembly through a predetermined range of motion, and a plurality of transmission elements positioned between the output and the input. The transmission elements are configured to provide a variable mechanical advantage between the output and the input during at least a portion of the predetermined range of motion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/256,255 filed on Apr. 18, 2014, which claims priority toU.S. Provisional Patent Application No. 61/813,813 filed on Apr. 19,2013 and U.S. Provisional Patent Application No. 61/898,790 filed onNov. 1, 2013. The entire contents of each of the foregoing applicationsare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to power tools, and more specifically tomagnetic drill presses.

BACKGROUND OF THE INVENTION

Magnetic drill presses perform drilling operations by latching amagnetic base of the drill press to a ferromagnetic workpiece. Suchmagnetic bases use electromagnets or permanent magnets for generating amagnetic field. A magnetic base with permanent magnets typicallyphysically reorient at least some of the permanent magnets to switch thebase between a latched configuration and a release configuration.

SUMMARY OF THE INVENTION

The invention provides, in one aspect, a magnetic drill press includinga main housing, a drill unit supported by the main housing for relativemovement therewith in a direction of a rotational axis of the drillunit, and a base coupled to the main housing for selectivelymagnetically latching to a ferromagnetic workpiece. The base includes amagnet holder, a fixed magnet assembly supported within the magnetholder, a rotatable magnet assembly supported within the magnet holder,and a transmission. The transmission includes an input, an outputcoupled to the rotatable magnet assembly for rotating the rotatablemagnet assembly relative to the fixed magnet assembly through apredetermined rotation angle, and a gear train positioned between theoutput and the input. The gear train is configured to provide a variablegear ratio between the output and the input during at least a portion ofthe predetermined rotation angle.

The invention provides, in another aspect, a magnetic drill pressincluding a main housing, a drill unit supported by the main housing forrelative movement therewith in a direction of a rotational axis of thedrill unit, and a magnetic base coupled to the main housing forselectively magnetically latching to a ferromagnetic workpiece. Themagnetic base includes a plurality of grooves formed on a surface of themagnetic base engageable with the ferromagnetic workpiece.

The invention provides, in another aspect, a magnetic base forselectively magnetically latching to a ferromagnetic surface. Themagnetic base includes a magnet holder, a fixed magnet assemblysupported within the magnet holder, a movable magnet assembly supportedwithin the magnet holder, and a transmission. The transmission includesan input, an output coupled to the movable magnet assembly for movingthe movable magnet assembly relative to the fixed magnet assemblythrough a predetermined range of motion, and a plurality of transmissionelements positioned between the output and the input. The transmissionelements are configured to provide a variable mechanical advantagebetween the output and the input during at least a portion of thepredetermined range of motion.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a magnetic drill press including amagnetic base in accordance with an embodiment of the invention.

FIG. 2 is a rear perspective view of the magnetic base of FIG. 1.

FIG. 3 is an exploded front perspective view of the magnetic base ofFIG. 2.

FIG. 4 is an exploded rear perspective view of the magnetic base of FIG.2.

FIG. 5A is a cross-sectional view of the magnetic base of FIG. 2 in alatching configuration taken along the line 5A-5A in FIG. 2.

FIG. 5B is a cross-sectional view of the magnetic base of FIG. 2 in arelease configuration.

FIG. 5C is a cross-sectional view of the magnetic base of FIG. 2 in thelatching configuration, magnetically latched to a workpiece.

FIG. 6 is a perspective view of a magnet holder of the magnetic base ofFIG. 2.

FIG. 7 is a front view of a gear train of the magnetic base of FIG. 2.

FIG. 8A is a perspective view of a transmission including the gear trainof FIG. 7, a rotatable magnet assembly coupled to the transmission, anda fixed magnet assembly of the magnetic base of FIG. 2, illustrating therotatable magnet assembly in a position coinciding with the latchingconfiguration of the magnetic base.

FIG. 8B is a perspective view of the transmission, the rotatable magnetassembly, and the fixed magnet assembly of FIG. 8A, illustrating therotatable magnet assembly in a position between the latchingconfiguration and the release configuration of the magnetic base.

FIG. 8C is a perspective view of the transmission, the rotatable magnetassembly, and the fixed magnet assembly of FIG. 8A, illustrating therotatable magnet assembly in a position coinciding with the releaseconfiguration of the magnetic base.

FIG. 9 is a chart comparing torque and gear ratio as a function ofrotation angle of the rotatable magnet assembly of FIG. 8A.

FIG. 10 is a chart comparing magnetic holding force as a function ofworkpiece thickness of the magnet holder of FIG. 2.

FIG. 11 is a rear perspective view of a battery receptacle of themagnetic drill press of FIG. 1.

FIG. 12 is a bottom perspective view of the magnetic drill press of FIG.1 including an adapter for mounting the magnetic drill press on anon-flat surface.

FIG. 13 is an exploded, bottom perspective view of the magnetic base andthe adapter of FIG. 12.

FIG. 14 is a cross-sectional view of the magnetic base and the adapterof FIG. 12.

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

DETAILED DESCRIPTION

FIG. 1 illustrates a magnetic drill press 10 including a drill unit 14,a main housing 18 to support the drill unit 14, and a magnetic base 22coupled to the main housing 18 and selectively magnetically latching themagnetic drill press 10 to a ferromagnetic workpiece (not shown). Thedrill unit 14 may include a DC motor or an AC motor to rotate a spindlewith a working tool 26 attached thereto about a rotational axis 28. Thedrill unit 14 is supported by the main housing 18 for relative movementtherewith in a direction along the rotational axis 28. The magneticdrill press 10 may be powered by a battery 30 as shown in theillustrated embodiment, from an AC voltage input (i.e., from a walloutlet), or by an alternative DC voltage input (e.g., a DC powersupply).

The battery 30 is selectively electrically connected with the drill unit14, and the main housing 18 includes a receptacle 31 in which thebattery 30 is at least partially received. In the illustratedembodiment, a majority of the battery 30 is contained within thereceptacle 31. With reference to FIG. 11, the receptacle 31 is definedby two side walls 146, a rear wall 150, a front wall 154, and a bottomwall 162. These walls 146, 150, 154, 162 protect and shield the battery30 from impacts, and in the illustrated embodiment, all but one side ofthe battery 30 is at least partially shielded within the receptacle 31.

With reference to FIG. 1, a power cord 32 extends between the mainhousing 18 and the drill unit 14 for delivering power from the battery30 to the drill unit 14. The power cord 32 forms a loop with a bottomportion 33 proximate the base 22. By routing the power cord 32 in thismanner, the overall height profile of the magnetic drill press 10 isreduced compared to other commercially available drill presses. Inaddition, by routing the power cord 32 in this manner, a user isdiscouraged from misusing the power cord 32 as a way to carry themagnetic drill press 10.

With reference to FIGS. 5A and 5B, the magnetic base 22 includes amagnet holder 34, a fixed magnet assembly 38 supported within the magnetholder 34, and a rotatable magnet assembly 42 supported within themagnet holder 34 and spaced from the fixed magnet assembly 38. In theillustrated embodiment, the magnet assembly 42 is rotatable with respectto the fixed magnet assembly 38, but in alternative embodiments themagnet assembly 42 may be movable with respect to the fixed magnetassembly 38 in other ways (e.g., translation, rotation and translation,etc.). The magnet holder 34 includes an end plate 46 to secure therotatable magnet assembly 42 and the fixed magnet assembly 38 within themagnet holder 34. With reference to FIGS. 5A-5B and 8A-8C, the rotatablemagnet assembly 42 in the illustrated embodiment includes a rotatabledrum 50 and four permanent magnets 54 (e.g., Neodymium magnets) affixedto the rotatable drum 50. The fixed magnet assembly 38 includes fourtrapezoidal permanent magnets 58 secured within the magnet holder 34 inthe illustrated embodiment. In alternative embodiments, the rotatablemagnet assembly and the fixed magnet assembly may each include one ormore permanent magnets. As illustrated in FIGS. 5A and 5B, thetrapezoidal permanent magnets 58 each include a north (N) and a south(S) pole that are laterally spaced with respect to a longitudinal axis62 of the magnetic base 22. Likewise, the permanent magnets 54 of therotatable magnet assembly 42 each include a north (N) and a south (S)pole that are laterally spaced with respect to the longitudinal axis 62in the positions shown in FIGS. 5A and 5B. With continued reference toFIGS. 5A and 5B, the magnet holder 34 includes bores 63 in which toreceive and store fasteners 64, the use of which is explained in furtherdetail below.

With reference to FIG. 6, the magnet holder 34 is shown with therotatable magnet assembly 42 and the fixed magnet assembly 38 removed tobetter illustrate the geometry of the magnet holder 34. The magnetholder 34 in the illustrated embodiment includes ridges 66 and grooves67 defined between adjacent ridges 66 formed on a bottom surface 70(i.e., the surface that engages the ferromagnetic workpiece). In theillustrated embodiment, the magnet holder 34 includes four ridges 66 andtwo grooves 67, the importance of which is described in greater detailbelow. In addition, the magnet holder 34 includes slots 74 formedbetween the rotatable magnet assembly 42 and the fixed magnet assembly38.

With reference to FIGS. 2-4, the magnetic base 22 further includes atransmission 78 having an input 82, an output 86, and a plurality oftransmission elements (e.g., a gear train 90) positioned between theoutput 86 and the input 82. As discussed in more detail below, the geartrain 90 is configured to provide a variable mechanical advantage (e.g.,a variable gear ratio) between the output 86 and the input 82. Thetransmission 78 also includes a cover 94 enclosing the gear train 90 andhaving an aperture 98 through which the input 82 extends. A hexagonalbody 102 and input knob 104 are coupled for co-rotation with thetransmission input 82, and the input knob 104 is accessible to the userof the magnetic drill press 10 for actuating the transmission 78. Theuser can apply torque to the transmission input 82 using the input knob104.

With reference to FIGS. 4, 7 and 8A-8C, the gear train 90 includes asingle gear stage with two non-circular gears 106, 110 that are meshed.In alternative embodiments, the gear train may include any number ofstages of circular and non-circular gears (e.g., oval-shaped,elliptical, etc.), including other means of torque transmission. In theillustrated embodiment, the two non-circular gears 106, 110 aredissimilar, however alternative embodiments can provide identicalnon-circular gears. The transmission input 82 is coupled for co-rotationwith a first of the non-circular gears 106, and the transmission output86 and the rotatable magnet assembly 42 are coupled for co-rotation witha second of the non-circular gears 110 (FIGS. 8-10).

With reference to FIG. 4, the input 82 and the first gear 106 arerotatably supported (e.g., by one or more bearings) by a support block122 secured to the magnet holder 34, and the first gear 106 is operableto rotate about a first, non-central axis 126 when an input torque isapplied by the user to the knob 104 and the transmission input 82. Thesecond gear 110 rotates about a second axis 130 parallel to the firstaxis 126. In alternative embodiments, the second axis 130 may be skewedor perpendicular to the first axis 126. A stop pin 134 extends from thesecond gear 110 at a location offset from the second axis 130. Withreference to FIG. 3, the stop pin 134 is received in an arcuate track138 to limit the rotation of the second gear 110, and therefore thetransmission output 86 and the rotatable magnet assembly 42, to apredetermined rotation angle (e.g., approximately 135 degrees in theillustrated embodiment). The track 138 is defined in the gear cover 94and spans an arc length nominally greater than 135 degrees to permit thesecond gear 110 to rotate approximately 135 degrees about the secondaxis 130. Therefore, the second gear 110 is limited to a predeterminedrotation angle of approximately 135 degrees about the second axis 130.In alternative embodiments, the arc length of the track may be differentdepending upon the predetermined rotation angle of the second gear 110(e.g., approximately 180 degrees). The variable gear ratio provided bythe first gear 106 and the second gear 110 exhibits an overall, averagegear ratio of 2:1 over the entire range of motion. Therefore, the inputgear 106 and the input knob 104 are limited to a range of rotation equalto approximately 270 degrees. In other words, in the illustratedembodiment, the second gear 110 rotates 135 degrees from start tofinish, which corresponds with the first gear 106 rotating 270 degreesfrom start to finish.

With reference to FIG. 4, the transmission output 86 is operable torotate the rotatable magnet assembly 42 relative to the fixed magnetassembly 38 through the predetermined rotation angle to actuate themagnetic base 22 between a first configuration (i.e., a latchedconfiguration, FIG. 5A) in which the base 22 can magnetically latch to aferromagnetic workpiece, and a second configuration (i.e., a releaseconfiguration, FIG. 5B) in which the base 22 cannot magnetically latchto the ferromagnetic workpiece. In the release configuration, themagnetic poles (N, S) of the rotatable magnet assembly 42 are positionedsuch that the north poles N of the rotatable magnet assembly 42 arediagonally offset from the north poles N of the fixed magnet assembly38, as shown in FIG. 5B. Similarly, the corresponding south poles S ofthe rotatable magnet assembly 42 and the fixed magnet assembly 38 arediagonally offset from each other. When in the release configuration,the magnetic fields of the rotatable magnet assembly 42 and the fixedmagnet assembly 38 are mostly contained within the magnet holder 34(i.e., almost none of the magnetic field flows through the workpiece,but rather is short-circuited within the magnet holder 34). With themagnetic base 22 in the release configuration, the user is able toposition the magnetic drill press 10 with respect to the ferromagneticworkpiece in preparation for a drilling operation. To latch the magneticbase 22 to a ferromagnetic workpiece, the magnetic base 22 is actuatedfrom the release configuration (FIG. 5B) to the latched configuration(FIG. 5A). In the latched configuration, the north and south poles N, Sof the rotatable magnet assembly 42 and the fixed magnet assembly 38,respectively, are oriented in the same direction (i.e., the north polesN of both the rotatable magnet assembly 42 and the fixed magnet assembly38 are on the same side of the base 22). When in the latchedconfiguration, the magnetic fields of the rotatable magnet assembly 42and the fixed magnet assembly 38 are directed externally of the magnetholder 34 and into the ferromagnetic workpiece.

With reference to FIG. 5C, lines of magnetic flux 140 are illustratedwith the magnetic base 22 in the latched configuration. When themagnetic flux 140 passes through a ferromagnetic workpiece in contactwith the magnet holder 34, a resultant holding force is establishedbetween the magnetic base 22 and the ferromagnetic workpiece. Thegrooves 67 (i.e., the absence of magnetic material) create a highdensity of magnetic flux in the ridges 66, which results in a highholding force present at the ridges 66. Including the grooves 67 on thebottom surface 70 thereby provide high magnetic flux density withcontrolled saturation in the ridges 66 to increase the holding force, incomparison to a similar magnetic base in which the bottom surface 70 isentirely flat. In particular, the holding force is increased forrelatively thin workpieces by including the grooves 67. With referenceto FIG. 10, the holding force is illustrated as a function of theworkpiece thickness for the magnetic base 22 with grooves 67, and asimilar magnetic base but without any grooves. As shown in a substantialpart of the range of FIG. 10, as the workpiece plate thicknessdecreases, the holding force for the magnetic base 22 with grooves 67 islarger than the magnetic base without any grooves. In other words, theridges 66, the grooves 67, and the slots 74 formed in the magnet holder34 adjust the magnetic characteristics (i.e., magnetic saturation) ofthe magnetic base 22 to improve the holding force between the magneticbase 22 and relatively thin ferromagnetic workpieces. With the magneticbase 22 latched or fixed to a ferromagnetic workpiece, the drill unit 14can be utilized, for example, to drill through the ferromagneticworkpiece.

Magnets exhibit repulsion when like poles (i.e., both north poles, orboth south poles) face each other and the strength of the repulsiondepends on the distance between the like poles. Changing the position ofa first magnet with respect to a second magnet will also change therepulsion created as the first magnet is moved from one position toanother. In the case of the magnetic base 22, the repulsion between likepoles of the rotatable magnet assembly 42 and the fixed magnet assembly38 impart a reaction torque or “magnet torque” on the transmissionoutput 86 as the rotatable magnet assembly 42 is rotated between therelease configuration and the latched configuration. The magnet torqueis loaded on the transmission output 86, and the load is reflectedthrough the transmission 78 to the transmission input 82 and the inputknob 104. In FIG. 9, the magnet torque is illustrated as a function ofthe degree of rotation of the rotatable magnet assembly 42.Specifically, FIG. 9 illustrates a range of rotation angle of 135degrees for the rotatable magnet assembly 42, and a range of rotationangle of 270 degrees for the input knob 104. As shown in FIG. 9, themagnet torque is variable over the 135 degree range of rotation angle.The spike in magnet torque followed by the decrease in the magnitude ofthe magnet torque at approximately 70 degrees is a result of thegeometric design and magnetic characteristics of the magnetic base 22.The magnet torque, if not accounted for in the manner described below ofthe invention, can cause the user to have to apply a large amount oftorque, possibly exceeding human capabilities, to rotate the rotatablemagnet assembly 42 between the release configuration and the latchedconfiguration.

With reference to FIGS. 8A, 8B, and 8C, the magnet holder 34 and thegear case 94 are removed to clearly illustrate the orientation of thefirst gear 106, the second gear 110, and the rotatable magnet assembly42. FIG. 8C illustrates the magnetic base 22 in the releaseconfiguration with the north poles N poles of the rotatable magnetassembly 42 diagonally offset from the north poles N of the fixed magnetassembly 38. In the release configuration, the magnetic fields from therotatable magnet assembly 42 and the fixed magnet assembly 38 are mostlycontained within the magnet holder 34; therefore, any holding forcegenerated by the magnetic base 22 while in this configuration isinsufficient to latch the base 22 to a ferromagnetic workpiece. As theuser applies torque to the transmission input 82 via the input knob 104and causes clockwise rotation (as viewed from FIG. 8C) of the first gear106 about the first axis 126, the meshed second gear 110 will rotatecounter-clockwise about the second axis 130. As the second gear 110rotates, the stop pin 134 rotates therewith in the track 138 defined inthe gear train cover 94. FIG. 8B illustrates the magnetic base 22 in atransitional configuration between the release and the latchedconfiguration, with the rotatable magnet assembly 42 rotatedapproximately 45 degrees. FIG. 8A illustrates the magnetic base 22 inthe latched configuration with the second gear 110 having traveled thepredetermined rotation angle (i.e., about 135 degrees) and the firstgear 106 having traveled the predetermined rotation angle (i.e., about270 degrees).

With reference to FIG. 9, the variable gear ratio of the gear train 90is illustrated as a function of the predetermined rotational angle ofapproximately 135 degrees (i.e., the rotational span of the rotatablemagnet assembly 42). As shown in FIG. 9, the gear ratio is designed toprovide a larger mechanical advantage (i.e., a large gear ratio) whenthe magnet torque is large, and to provide a smaller mechanicaladvantage (i.e., a small gear ratio) when the magnet torque is small.With reference to FIG. 7, the meshed gears 106, 110 form a line ofcontact 142 between the rotational axes 126, 130 of the gears, and theradius of each of the gears 106, 110 continuously changes duringrotation. In other words, the gear ratio is the smallest when the firstgear 106 radius R1 along the line of contact 142 is the largest and thesecond gear 110 radius R2 along the line of contact 142 is the smallest.Likewise, the gear ratio is the largest when the first gear 106 radiusR1 along the line of contact 142 is the smallest and the second gear 110radius R2 along the line of contact 142 is the largest.

The gear train 90 is configured to apply a variable output torque to therotatable magnet assembly 42 through at least a portion of thepredetermined rotation angle of the rotatable magnet assembly 42. Thevariable gear ratio generally increases when the magnet torqueincreases, and generally decreases when the magnet torque decreases. Inaddition, as described above, the variable gear ratio has an overall,average 2:1 gear ratio over the entire range of motion. With referenceto FIG. 9, the variable gear ratio is designed to follow the magnettorque so that the torque input required by the user is generallyconstant (i.e., within a band of about 5 N-m) during a substantialportion (i.e., at least about 75%) of the predetermined rotation angleof the rotatable magnet assembly 42 between the release configurationand the latched configuration. More specifically, the torque inputrequired by the user is within a band of about 2.5 N-m during at leastabout 64% of the predetermined rotation angle. The first gear 106 meshedwith the second gear 110 in the illustrated embodiment provides thevariable gear ratio (i.e. R2/R1) with a range between about 1:1 andabout 4:1, and more specifically between about 1.4:1 and about 3.5:1.Alternatively, the variable gear ratio may have any range, includingbetween about 1:2.4 and about 2.4:1, and between about 1:9 and about9:1. In other words, the gear train 90 is configured to receive an inputtorque from the user for rotating the rotatable magnet assembly 42through the predetermined rotation angle, and the input torque is keptwithin human capabilities without large spikes in required input torquethroughout the entire range of the predetermined rotation angle. Inalternative embodiments, the transmission elements can provide any rangeof gear ratios, including reduced ratios (e.g., 1:2).

The transmission 78 creates an improved feel for the user as themagnetic base 22 is switched between the latched configuration and therelease configuration. In contrast, prior art magnetic bases required aspike in input torque by the user to account for the non-linear magnettorque. This spike in required input torque creates an unstable feel forthe user and can exceed the user's physical ability. The magnetic base22 of the invention eliminates these problems by making the input torqueapplied by the user generally constant throughout a substantial range ofrotation angle of the rotatable magnet assembly 42, thereby eliminatingany spikes in required input torque and keeping the applied input torquewithin a user's physical ability. In addition, not only is the requiredinput torque kept within a user's physical ability with the transmission78, but the amount of required input rotation is also reduced.Commercially available drill presses utilize a constant gear ratio(e.g., 5:1) which keeps the required input torque low, but requires arelatively large amount of input rotation by the user (e.g., 675degrees). In other words, the transmission 78 simultaneously reduces therequired input torque and the required input rotation.

In alternative embodiments, the transmission input 82 can be rotated byan amount that is independent (i.e., different) than that of thetransmission output 86 for actuating the magnetic base 22 between thelatched and release configurations (i.e., the transmission input 82 canbe rotated more or less than the required transmission output 86rotation). For example, to make the user's operation of switching themagnetic base 22 between the latched configuration and the releaseconfiguration intuitive, the knob 104 and transmission input 82 can berotated about 180 degrees to actuate the magnetic base 22 between thelatched and release configurations. In alternative embodiments, a motorcan be included to switch the magnetic base between the releaseconfiguration and the latched configuration instead of requiring userinput torque and rotation. The motor can be advantageously selected tobe efficient and small as a result of the variable mechanical advantageprovided by the transmission 78. In other words, since the motor needsonly to be designed to handle a uniform load profile (i.e., a relativelyconstant torque), the design specifications for the motor becomesimplified.

In the illustrated embodiment, the transmission input 82 is rotatedclockwise (as viewed from FIG. 8C) to switch the magnetic base 22 fromthe release configuration to the latched configuration. However, inother embodiments, the gear train 90 may be configured such that thetransmission input 82 is rotated counter-clockwise to switch themagnetic base 22 from the release configuration to the latchedconfiguration. In further alternative embodiments, the variable gearratio could be designed to exactly match the variable magnet torque,resulting in an input torque held within a band of 1 N-m or less acrossa substantial portion of rotation angle of the rotatable magnet assembly42. In general, in alternative embodiments, the transmission can becustomized to provide any user experience regarding the required inputtorque (i.e., gradually increasing input torque, gradually decreasinginput torque, oscillating input torque, etc.). In alternativeembodiments, the transmission could include any number and configurationof transmission elements (e.g., linkages, gears, cams, etc.) thatprovide a variable mechanical advantage between the transmission input82 and the transmission output 86.

With reference to FIGS. 12-14, the drill press 10 may also include anadapter 200. In some applications, it is desired to use the magneticdrill press 10 on a non-flat, or curved, surface (e.g., a pipe P). Theadapter 200 is removably coupled to the magnet holder 34 to facilitateplacement of the magnet holder 34 on a curved ferromagnetic workpiece.The adapter 200 includes a plurality of projections 204 received withinthe corresponding grooves 67 in the magnet holder 34 (FIG. 13). Theadapter 200 is secured to the magnet holder 34 with the fasteners 64which, in turn, are threaded within the bores 63. The adapter 200includes a first side portion 208 and a second side portion 212, andeach of the side portions 208, 212 includes a curved surface 218 that isengageable with the curved ferromagnetic workpiece. A window 222 isdefined between the first side portion 208 and the second side portion212.

In the illustrated embodiment, the window 222 is unobstructed; however,alternative embodiments may include non-ferromagnetic material withinthe window. When the adapter 200 is removed from the magnet holder 34and is not in use, the fasteners 64 may be stored within the bores 63 inthe magnet holder 34 until the adapter 200 is attached again. The curvedsurfaces 218 provide at least two points of contact on which to supportthe magnet holder 34 on workpieces having a variety of curvature. Inother words, the curved surfaces 218 allow the magnet holder 34 to besupported on a range of pipe diameters, for example. In alternativeembodiments, the adapter may include a non-metal (e.g., plastic) frameand a plurality of ferromagnetic blocks secured together by the frame.The blocks may further include a curved surface that is engageable withthe curved ferromagnetic workpiece.

Various features of the invention are set forth in the following claims.

What is claimed is:
 1. A magnetic base for selectively magneticallylatching to a ferromagnetic surface, the magnetic base comprising: amagnet holder; a fixed magnet assembly supported within the magnetholder; a movable magnet assembly supported within the magnet holder;and a transmission including an input, an output coupled to the movablemagnet assembly for moving the movable magnet assembly relative to thefixed magnet assembly through a predetermined range of motion, and aplurality of transmission elements positioned between the output and theinput, the elements configured to provide a variable mechanicaladvantage between the output and the input during at least a portion ofthe predetermined range of motion.
 2. The magnetic base of claim 1,wherein the movable magnet assembly is rotatable relative to the fixedmagnet assembly.
 3. The magnetic base of claim 2, wherein thepredetermined range of motion of the movable magnet assembly is apredetermined rotation angle, and wherein the movable magnet assembly isrotatable relative to the fixed magnet assembly through thepredetermined rotation angle.
 4. The magnetic base of claim 3, whereinthe moveable magnet assembly is rotated about 135 degrees relative tothe fixed magnet assembly to actuate the magnetic base between a firstconfiguration in which the magnetic base can magnetically latch to aferromagnetic workpiece, and a second configuration in which themagnetic base cannot magnetically latch to the ferromagnetic workpiece.5. The magnetic base of claim 4, wherein the transmission input isrotated about 270 degrees to actuate the magnetic base between the firstand second configurations.
 6. The magnetic base of claim 3, wherein theplurality of transmission elements includes a gear train that provides avariable gear ratio between the output and the input during at least aportion of the predetermined rotation angle.
 7. The magnetic base ofclaim 6, wherein the gear train includes two non-circular gears that aremeshed.
 8. The magnetic base of claim 7, wherein the transmission inputis coupled for co-rotation with a first of the non-circular gears, andwherein the transmission output is coupled for co-rotation with a secondof the non-circular gears.
 9. The magnetic base of claim 6, wherein thegear train is configured to apply a variable output torque to themoveable magnet assembly through at least a portion of the predeterminedrotation angle of the moveable magnet assembly, wherein the variablegear ratio generally increases when the variable output torqueincreases, and wherein the variable gear ratio generally decreases whenthe variable output torque decreases.
 10. The magnetic base of claim 9,wherein the gear train is configured to receive an input torque from theuser for rotating the moveable magnet assembly through the predeterminedrotation angle, and wherein the input torque is within a band of about 5N-m during at least 75% of the predetermined rotation angle.
 11. Themagnetic base of claim 6, further comprising a stop in the gear trainfor limiting the predetermined rotation angle.
 12. The magnetic base ofclaim 6, wherein the variable gear ratio includes a range between about1:2.4 and about 2.4:1.
 13. The magnetic base of claim 1, wherein themagnet holder includes a slot between the moveable magnet assembly andthe fixed magnet assembly.
 14. The magnetic base of claim 1, wherein themoveable magnet assembly includes a rotatable drum and a plurality ofpermanent magnets affixed to the rotatable drum.
 15. The magnetic baseof claim 1, wherein the magnet holder includes ridges on a surface ofthe magnet holder engageable with the ferromagnetic workpiece.
 16. Themagnetic base of claim 15, wherein the magnet holder includes a groovedefined between adjacent ridges, and wherein the groove is a first of aplurality of grooves.
 17. The magnetic base of claim 16, furthercomprising an adapter coupled to the magnet holder to facilitateplacement of the magnet holder on a curved ferromagnetic workpiece. 18.The magnetic base of claim 17, wherein the adapter includes a pluralityof projections received within the corresponding plurality of grooves inthe magnet holder.
 19. The magnetic base of claim 17, wherein theadapter includes a first side portion and a second side portion, andwherein each of the first and second side portions includes a curvedsurface that is engageable with the curved ferromagnetic workpiece. 20.The magnetic base of claim 19, wherein the adapter includes a windowdefined between the first side portion and the second side portion.